Get relaxed. When warming up your voice you want to be relaxed, so it's probably best that you do a couple stretches so your body is loose and you are ready to warm up your voice.
Start using the lower register of your voice and use the vowels "oo" first. Go as low as you can possibly go with your voice. Not only will it help you reach a higher intensity and register, but it will also help you improve your lower range at the same time.
Mimic the sound of the siren by going from your low vocal register to the highest part of your voice. The vowels and sound that you make should resemble "wooah wwoah wooah," that of a siren.
Reach the absolute pinacle of where your voice can go in terms of pitch, after you have reached this pinacle, stay up for there for a while by mimicking the siren sound, but come back down slowly to the beginning note of your vocal exercise.